Did you know that only half of US gardeners (and one third of British gardeners!) compost?
Given that compost is incredibly good for the soil, that’s quite a surprising statistic!
And I think one reason why so many gardeners don’t compost is because composting is perceived to be difficult.
So in the latest of our series of articles on lazy compost techniques, let’s take a look at one of the easiest ways to make compost – plastic bag composting.
What is plastic bag composting?
Plastic bag composting (also known as garbage bag composting) is exactly as the name describes.
This method involves placing organic material into a plastic bag and allowing it to decompose over time, ultimately transforming into compost.
As we’ll see, while this method does have its drawbacks, it is a very low-maintenance way to make compost.
How does it work?
To understand how plastic bag compost works, we need to understand the difference between aerobic and anaerobic compost. Let’s take a brief look at both of these:
Aerobic (with air) composting: In this process, bacteria, microorganisms and physical decomposers which thrive in an oxygen-rich environment break down organic material into compost.
Anerobic (without air) composting: In this process, microorganisms that thrive in a limited-oxygen environment do the bulk of the breaking-down process.
The Process of Plastic Bag Composting: This method of composting is largely an anaerobic process, as most of the air is excluded from the material.
When organic material is enclosed in a plastic bag, the restricted oxygen supply creates an environment where anaerobic microbes can thrive.
These microbes then gradually decompose the organic material, transforming it into compost.
As with aerobic composting, the microbes in anaerobic composting need both nitrogen and carbon, so it’s important to include both high-nitrogen and high-carbon sources.
Let’s take a look at those materials next!
The materials that can be used for trash bag composting
Most organic materials can be used. However, as mentioned, you do need to include both high-nitrogen and high-carbon materials.
Here’s some examples:
|High Carbon (Brown) materials
|High Nitrogen (Green) materials
Straw or hay
Wood chips or bark
Dried ornamental grasses
Fresh grass clippings
Weeds (without seeds)
Seaweed and kelp
Green plant clippings
Materials to avoid
Meat, fish, cooked food: These can attract pests which will break open the plastic bag.
Sticks and twigs: While these can theoretically be composted, their high lignin content means they can take a long time to break down, and can make holes in the edge of the bag.
Cat and dog droppings: These can contain pathogens that may or may not be killed in the anaerobic process (see FAQs for more information about anaerobic composting and pathogens). These are best kept for hot composting or bokashi composting.
Weeds with seeds: Some seeds will survive the composting process, so if this is a concern to you, they are best left out.
Let’s do it
What do you need
- Sturdy garbage bags (I used rubble sacks for the inner bag)
- Some soil or compost (used compost is fine)
- Organic materials, both Greens and Browns
- Start with a layer of compost or soil
There are two reasons for this.
First, the soil or compost should introduce the microorganisms you need. Composters are often split on whether this is necessary, but some research suggests it helps, and it doesn’t take much effort to add.
Secondly, it should help absorb some moisture trickling down through the compost.
- Add a layer of brown (high carbon) materials.
As with the above, I add sawdust before adding green materials to create another layer to absorb moisture.
You don’t need to add sawdust, though, any high-carbon material will do.
To learn more about high carbon and high nitrogen materials see our article on the Carbon: Nitrogen ratio.
- Add a layer of green material
I’ve added a layer of grass here, as it very high in nitrogen. (Plus, I’d just cut the lawn ;))
- Continue layering until full.
(I also added a bit of manure in for good measure.)
- Add water
The added water forces out any trapped oxygen, further facilitating the anaerobic process.
- Tie the bag up, and double bag it.
This should prevent splitting, and help prevent air from getting in.
- Ideally, place in a warm place.
One study suggests that the optimum temperature for anaerobic composting is 35 degrees Celsius.
While that was looking at biogas production, the study also found that for each 10celsius drop in temperature, the activity and reproduction of bacteria dropped by half.
You might not have a warm place available, and that’s fine – the process will just take longer.
Let’s split-test it
I couldn’t help wondering how important the carbon was in this process.
So I’m currently split-testing the process. I have one bag filled with the traditional method, and one with just grass and a little bit of compost and manure to add microbes.
It’ll probably be a soggy mess at the end, but we’ll see!
Avoid puncturing the bag
Try hard to avoid puncturing the bag, as the liquid that accumulates in the bottom can be foul!
Put in a rollable bin
Carry on Composting also recommends placing the bags in a bin and rolling it every couple of weeks to mix the materials.
If you’re choosing this method for ease of use, this may not be for you, but if you want to maximize the efficiency of the process, it’s worth a try!
Simple and easy
While you still have to think about the materials you add to the bag, the process requires zero maintenance.
Repurpose used compost
Used compost can be a useful source of carbon, and this can be a good way to repurpose it into fresh compost, enriched with nutrients from the other materials you use.
Free or cheap
Plastic bags are very cheap, and you may even be able to reuse some existing bags.
Great for winter composting
If you live in a place with a really cold winter, composting can be hard. The ground may be frozen, and the compost heap covered in snow. In this situation, preparing a bag might be easier, and it can then be dropped in a bin or a shed until spring.
Losing environmental benefits
One of the reasons environmentalists want us to compost our waste is to divert it from landfills. In landfills, organic waste breaks down anaerobically, which releases methane.
Plastic bag composting is also an anaerobic process, which means it will also release methane.
Anaerobic composting takes a lot longer than aerobic composting – in fact, it can take 6 months longer!
The limitation of using plastic bags means there is only so much compost that you can make! If you need to create large amounts of compost, you are better off with a three-bin compost system or a heap.
Possibly lower quality
One recent study found that compost that did not get enough oxygen, especially in the early days of composting, was of lower quality than compost made with sufficient aeration. See The Importance of Oxygen in the Early Stages of Composting for details.
ypically, this is the point where I’d encourage you to give plastic bag composting a try.
However, it’s important to consider the potential drawbacks that come with this method.
One of the biggest is that plastic bag composting produces methane, which is harmful to the environment, and the compost might be of lower quality than compost made in more traditional ways.
That said, there are times when it is useful.
For example, if you are in the middle of a harsh winter, and it’s not feasible to make compost in other ways, this might be the best choice for you.
After all, there are many ways to compost, and plastic bag composting is another option on the menu which you can pick when it is right for you!
Does plastic bag composting kill pathogens?
While some sources state that the harsh anaerobic conditions kill pathogens in plastic bag composting, sources are rarely given for this.
What’s more, there is little research into plastic bag composting.
Some studies do suggest that pathogens are killed in anaerobic composting conditions. For example, Termorshuizen et al found that many (but not all) pathogens are inactivated by anaerobic composting. However, the conditions used in research experiments may differ from those in home composting.
Does plastic bag composting kill weed seeds?
No, or at least not all of them! Studies have found that even in commercial anaerobic composting some weeds survive and are all viable.
The Mini Farming Guide to Composting (Amazon link) covers anaerobic composting processes in chapter 5. In addition to plastic bag composting, the chapter covers buried composting, anaerobic container composting and open pile anaerobic composting.